1. Field of the Invention
This invention relates to a method of forming crystals, particularly to the method of forming crystals by utilizing the difference in nucleation density in liquid phase.
The present invention may be applied for formation of crystals including monocrystals, polycrystals, etc. to be used for, for example, electronic devices, optical devices, magnetic devices, piezoelectric devices or surface acoustic devices, etc. of semiconductor integrated circuit, optical integrated circuit, magnetic circuit, etc.
2. Related Background Art
In the prior art, monocrystalline thin films used for semiconductor electronic devices, optical devices, etc. have been formed by epitaxial growth on a monocrystalline substrate. For example, on a Si monocrystalline substrate (silicon wafer), Si, Ge, GaAs, etc. have been known to be epitaxially grown from liquid phase, gas phase or solid phase, and also on a GaAs monocrystalline substrate, monocrystals of GaAs, GaAlAs, etc. have been known to be epitaxially grown.
Particularly, when Si is epitaxially grown from liquid phase onto Si monocrystal, a solution comprising Si which is the depositing material dissolved in Ga, In, Sb, Bi, Sn, etc. as the solvent is used.
Concerning such Si epitaxial growth, there are reports in many literatures as shown below.
B. Girault, F. Chevrier, A. Joulle and G. Bougnot, J. Crystal Growth 37, 169 (1977); PA0 D. Kass, M. Warth, W. Appel, H. P. Strunk and E. Bauser, Electrochemical Society, Meeting 1985; PA0 B. J. Baliga, Journal of Electrochemical Society, vol. 126, P. 138, 1979 PA0 B. J. Baliga, Journal of Electrochemical Society, vol. 124, P. 1627, 1977; PA0 B. J. Baliga, Journal of Crystal Growth, vol, 41, P. 199, 1977; PA0 B. J. Baliga, Journal of Electrochemical Society, vol. 125, P. 598, 1978.
All of the above literatures use liquid phase epitaxial growth on monocrystalline Si substrates, not on other materials than monocrystalline Si substrates such as amorphous SiO.sub.2.
By use of semiconductor thin films thus formed, semiconductor devices and integrated circuits, and emission devices such as semiconductor lasers, LED, etc. are prepared.
Also, in recent years, research and development have been abundantly conducted on ultra-high speed transistors by use of two-dimensional electron gas, superlattice device utilizing quantum well, etc., and these have been made possible by high precision epitaxial technique such as MBE (molecular beam epitaxy) by use of ultra-high vacuum, MOCVD (metal organic chemical vapor deposition), etc.
In such epitaxial growth on a monocrystalline substrate, it is required to adjust the lattice constant and coefficient of thermal expansion between a monocrystalline material of the substrate and an epitaxial growth layer. For example, although it is possible to grow epitaxially a Si monocrystalline thin film on sapphire which is an insulating monocrystalline substrate, the crystal lattice defect at the surface due to deviation in lattice constant and diffusion of aluminum which is the component of sapphire into the epitaxial layer, etc. are becoming problems in application to electronic devices or circuits.
Thus, it can be understood that the method of forming a monocrystalline thin film of the prior art according to epitaxial growth depends greatly on its substrate material. Mathews et al. examined the combinations of the substrate materials with the epitaxial growth layers (EPITAXIAL GROWTH, Academic Press, New York, 1975 ed. by J. W. Mathews).
Also, the size of the substrate is presently about 6 inches for Si wafer, an enlargement of GaAs, sapphire substrate has been further delayed. In addition, since the production cost of a monocrystalline substrate is high, the cost per chip becomes high.
Thus, for forming a monocrystalline layer capable of preparing a device of good quality according to the process of the prior art, there has existed a problem in that the kinds of the substrate material are limited to an extremely narrow scope.
On the other hand, in recent years, research and development has been extensively done on the three-dimensional integrated circuits formed by laminating semiconductor devices in the normal direction of a substrate to achieve a highly integrated and polyfunctional state. Research and development is also extensively being made year by year on large area semiconductor devices in which elements are set in an array on an inexpensive glass, such as solar batteries and switching transistors for liquid crystal picture elements.
What is common to such research and development is that they require techniques by which a semiconductor thin film is formed on an amorphous insulating material and an electronic device such as a transistor is formed thereon. Particularly sought after among these is a technique by which a monocrystalline semiconductor of high quality is formed on an amorphous insulating material.
In general, however, the deposition of a thin film on the amorphous insulating material such as SiO.sub.2 may generally make amorphous or polycrystalline the crystalline structure of the deposited film because of lack of long-distance order of the substrate material. Here, the amorphous film refers to a film kept in a state that the short-distance order as in most vicinal atoms is retained but there is no long-distance order beyond that, and the polycrystalline film refers to a film in which monocrystalline grains having no particular crystal direction have gathered together separated at the grain boundaries.
For example, when Si is formed on SiO.sub.2 by the CVD method, if the deposition temperature is about 600.degree. C. or lower, an amorphous silicon is formed, while at a temperature higher than that, a polycrystalline silicon with grain sizes distributed between some hundred to some thousand .ANG. is formed. However, the grain size and its distribution will vary greatly depending on the formation method.
Further, a polycrystalline thin film with a large grain size of about micron or millimeter is obtained by melting and solidifying an amorphous or polycrystalline film with an energy beam such as laser, rod-shaped heater, etc. (Single-Crystal silicon on non-single-crystal insulators, Journal of Crystal Growth vol. 63, No. 3, October, 1983, edited by G. W. Cullen).
When transistors are formed on thin films of various crystal structures thus formed, and electron mobility is measured from its characteristics, a mobility of ca. 0.1 cm.sup.2 /V.sec is obtained for amorphous silicon, a mobility of 1 to 10 cm.sup.2 /V.sec for a polycrystalline silicon having a grain size of some hundred .ANG., and a mobility to the same extent as in the case of monocrystalline silicon for a polycrystalline silicon with a large grain size by melting and solidification.
From these results, it can be understood that there is great difference in electrical characteristics between the device formed in the monocrystalline region within the crystal grain and the device formed as crossing over the grain boundary. In other words, the deposited film on the amorphous material becomes amorphous or a polycrystalline structure and the device prepared there is greatly inferior in performance as compared with the device prepared in the monocrystalline layer. For this reason, uses are limited to simple switching device, solar battery, photoelectric converting device, etc.
Also, the method for forming a polycrystalline thin film with a large grain size by melting and solidification had the problem that enormous time is required for making grain size larger, because an amorphous or monocrystalline thin film is scanned with an energy beam for each wafer, whereby bulk productivity is poor and the method is not suited for enlargement of area.
As described above, in the method of growing crystals of the prior art and the crystals formed thereby three-dimensional integration or enlargement of area cannot be easily conducted to devices for practical applications whereby crystals such as monocrystals and polycrystals required for preparation of a device having excellent characteristics could not be formed easily and at low cost.